The present invention relates to a magnetic recording medium such as a magnetic tape, a magnetic disk, a magnetic card, etc., and particularly to a magnetic recording medium of a thin film type suitable for super high density recording.
Recently, with the digitalization of audio and video information, with the development and advancement in the form of multimedia, of information processing devices, and with the construction and advancement of information networks including those information processing devices, the recording media for the information processing devices have been requested to improve their performances such as recording capacity, recording/reproducing speed, cost, miniaturization, reliability, and so on.
Particularly, a magnetic disk unit is an external storage unit suitable for high density recording. As a magnetic recording medium used in such a magnetic disk unit, a coating-type magnetic recording medium in which powder of oxide magnetic substances is applied onto a support, and a thin-film-type magnetic recording medium in which a thin film of metal magnetic substances is made up on a substrate by evaporation, sputtering or the like have been known.
The thin-film-type magnetic recording medium is more suitable for higher recording density because of its higher density of a magnetic layer than that of the coating-type magnetic recording medium, so that it has often been used in hard magnetic disk drive units or portable fixed magnetic disk drive units (removable hard disk units).
As for a general structure of a hard magnetic disk which is a thin-film-type magnetic recording medium used in such a fixed magnetic disk drive unit as described above, a structure in which a subbing layer, a magnetic layer and a protective layer are formed sequentially on a substrate is well known. In order to increase the recording storage capacity of the fixed magnetic disk unit, it is necessary not only to increase the magnetic coercive force of the thin-film-type magnetic recording medium, but also to reduce the product of the residual magnetization value of the magnetic recording medium and the thickness of the magnetic layer so called Br.times.d in order to reduce a demagnetizing field from a bit boundary, and it is further necessary to reduce media noise.
Conventionally, an Al-Mg alloy has been often used for a substrate of the thin-film-type magnetic recording medium used in the hard magnetic disk drive unit. An NiP-plated layer is formed on the surface of the disk-like substrate and polishing is then given thereto so as to eliminate faults in the substrate surface which will cause errors in recording/reproducing. Further, an extremely minute streak pattern (groove) or moderate roughness is given to the polished substrate surface by textured finish so as to reduce a friction coefficient between the magnetic disk and a magnetic head at the time of CSS operation (Contact Start Stop), and so as to improve magnetic characteristics in the circumferential direction of the magnetic disk, particularly a magnetic coercive force Hc, a squareness ratio S, and magnetic coercive force squareness ratio S*.
In addition, in the case of the portable hard magnetic disk drive unit, the request of impact resistance of the magnetic disk is severe, and any substrates formed of glass, copper, titanium, zirconia, calcium oxide, carbon, silicon, etc. other than the AL-Mg alloy substrate, may be used.
In a manufacturing method of the thin-film-type magnetic recording medium for the hard magnetic disk, generally, a subbing film layer of Cr or the like is first formed on the substrate by sputtering process; a thin magnetic film is then formed by sputtering process as a magnetic layer such as CoCrPt, CoCrTa or the like; a protective film layer is formed by sputtering process with, for example, carbon, or by forming diamond-like carbon (DLC) by plasma CVD or the like; and, finally, a lubricating layer of fluorocarbon such as perfluoropolyether or the like is applied to thereby complete the thin-film-type magnetic recording medium.
In order to realize high recording density, on the other hand, as for a magnetic disk unit, a magneto-resistance effect type magnetic head (MR head) has been developed. As a problem of head to media interface, it is necessary to reduce distance between a recording medium and a magnetic head. In the case of a hard magnetic disk drive unit, reduction of the floating quantity of a magnetic head is tried. In this case, however, the probability that the magnetic head contacts with the magnetic disk becomes high. In the case of a floppy disk unit or the like, flexible supports such as high polymer films or the like are used, but the flatness of these supports is generally lower than that of the above-mentioned Al-Mg alloy substrate or the like, so that the frequency that the magnetic head and the magnetic disk slide on each other becomes high, and it is also considered that they always slide according to circumstances.
Further, because of the demand of a high recording storage capacity, small in the size, low in the cost and portable, a flexible thin-film-type magnetic recording medium in which a thin film of a metal magnetic substance is formed on a high polymer film having a thickness in a range of from 10 .mu.m to 100 .mu.m used as a support is carried out. This medium is expected to have a performance exhibiting both the characteristics such as impact resistance, portability, lightness in weight (weight of a recording medium body and a rotating mechanism), etc. possessed by a floppy disk, and the characteristics such as high recording storage density, low noise characteristic, etc possessed by a thin-film-type magnetic recording medium.
Some methods have been considered to manufacture a flexible thin-film-type magnetic recording medium using, as a support, a high polymer film made from polyethylene terephthalate (PET), polyethylene naphthalate (PEN), etc. There are, for example, a method in which such a high polymer film is punched into a disk in advance, and then, film formation is made by a disk holder type sputtering apparatus in the same manner as in the case of a conventional hard magnetic disk in a state where the disk is set in a substrate holder; and a pallet passing type sputtering method in which a plurality of substrate holders are mounted on a pallet, and passed on a sputtering target having a large area to thereby preform film formation. However, these methods have a problem on the suitability for mass production as well as a problem on the cost. Other than these methods, there is a web carriage continuous film forming method as a method which is high in productivity and advantageous in cost.
Generally, in a manufacturing method of a thin-film-type magnetic medium for a hard magnetic disk (hard disk), in order to increase the magnetic coercive force Hc and reduce the medium noise, substrate heating conditions are important in addition to the above mentioned polishing or textured-finishing of the Al-Mg alloy substrate (NiP-plated). That is, by heating the substrate, crystal grains constituting a magnetic film can be separated from each other magnetically, and the size of the crystal grains can be reduced.
In addition, the reason why a metal thin-film-type medium of a hard magnetic disk (hard disk) has a double-layer structure of a subbing layer such as Cr or the like and a magnetic layer such as CoCrPt, CoCrTa or the like is to increase the in-plane magnetic coercive force of the Co alloy thin film. The reason can be explained as follows from the point of metal crystallographic view. A Co alloy thin film is generally apt to be a thin film having an axis of easy magnetization in the vertical direction of the film. On the other hand, if a Cr thin film is formed on a substrate and a Co alloy thin film is formed directly thereon under certain film forming conditions, Cr and Co alloy are so close in lattice constant that the Co alloy film grows up on the Cr film hetero-epitaxially. As a result, the easy-magnetization axis of the Co alloy thin film grows up with an inclination toward the direction of the substrate surface, so that a high magnetic coercive force can be generated in the surface. That is, the Cr subbing layer has a function to characterize the magnetic characteristics such as the in-plane magnetic coercive force Hc, etc. of the Co alloy thin film, and it is important to establish the film-forming conditions properly.
On the other hand, when a web-like high polymer film is used as the support as mentioned above, it is difficult to give polishing or circumferentially textured-finishing to respective disks one by one. In addition, the heating temperature is limited according to the kind of the high polymer support. Therefore, when a web-like high polymer film is used as the support to make up a flexible thin-film-type magnetic recording medium, a conventional manufacturing method of a thin-film-type magnetic recording medium for a hard magnetic disk cannot be put into practical use as it is.
The present inventors tried to make up a flexible thin-film-type magnetic recording medium by a method in which a subbing film layer such as Cr and a magnetic film layer such as CoCrPt were formed on a continuous web by sputtering process so as to form a thin magnetic film, while a high polymer film was carried as the web.
Specifically, by use of a continuous sputtering apparatus having a uncoil/coil apparatus, a high polymer film such as polyethylene terephthalate, polyethylene naphthalate or the like having a thickness in a range of from 10 .mu.m to 200 .mu.m was carried as a continuous web, while the web was made to be opposite to a sputtering target and carried in a space (there was no supporting mechanism on the back of the web formed by sputtering process). In such a state, a Cr subbing layer was formed in the form of a film by sputtering and, succeedingly, a CoPtCr magnetic layer was formed in the form of a film by sputtering in a similar state of the web.
However, it was found that cracks were apt to be occurred in the thin film on the high polymer film in the above-mentioned sputtering process of the subbing film. As a result, cracks were also occurred in the magnetic layer formed on the subbing layer, so that it was difficult to make up a flexible thin-film-type magnetic recording medium.